1. Field of the Invention
This invention relates to a method and composition for treating the surface of unset Portland cement. In particular, this invention relates to a method and composition for forming a hardened surface of high compressive strength and possessing a high degree of fluid impermeability while exhibiting an increase in surface hardness and abrasion resistance and early strength by dry shaking, that is, sprinkling, a mixture of aluminous cement and gypsum on the unset surface of Portland cement concrete. More specifically, the present invention relates to a surface treating method utilizing a dry cementitious composition containing aluminuous cement and gypsum sprinkled on unset Portland cement concrete. The composition may optionally contain additives.
The terms "cementitious compositions", or "cementitious systems" as used herein, are intended to mean compositions which generally possess the characteristic of hardening under water including cementitious waterproofings, toppings, protective coatings, and the like as well as mixtures with aggregates and water such as concrete, mortar, grout and products made therefrom.
The term "aluminous cement", as used herein, is intended to include those cementitious materials normally understood in the art to contain, as the main cementitious constitutent, mono-calcium aluminate (CaO:Al.sub.2 O.sub.3), such as high alumina cement and calcium aluminate cement. Examples of commercially available aluminous cements suitable for use in the invention are Lumnite by Lehigh Portland Cement Company and the high alumina cement Fondu or Secar by Lonestar LaFarge Aluminous Cement Company, Ltd. Some commercially available aluminous cements may have had gypsum added to them, in preparation of the clinker before fixing. This is added solely as an aid to manufacturing and is belived to result in a SO.sub.3 content in the cement equal to or less than 2.5 percent. Gypsum has been added, in small quantities, to aluminuous cements after firing as a set controller.
The term "gypsum", as used herein, is intended to include gypsum such as normally understood in the art, including calcium sulfate (CaSO.sub.4) and its various forms, calcium sulfate anhydrate, calcium sulfate hemidydrate, calcium sulfate dihydrate as well as calcined gypsum, pressure calcined gypsum and plaster of Paris. Examples of commercially available gypsums suitable for use in the invention are the high pressure calcined gypsum sold under the tradename "Densite", "K-5" and "K-34", by Georgia Pacific Corporation or the tradename "Hydrostone" by United States Gypsum Company.
The term "Portland cement", as used herein, is intended to include those cements which have sufficient "Portland cement" in them for interaction with the surface treating compositions and includes those normally understood in the art to be "Portland cements" including, but not limited to, those described in the designation ASTM C 150 as well as Portland cement-based cementitious compositions such as expansive cements, pozzolanic cement, mixtures of Portland cement with other cements and blends of Portland cement. The term "concrete", as used herein, is intended to include a mixture of cement, aggregate and water. The term "unset", as used herein, is intended to mean any time before final set. Generally, in the field this means a time when one can get on the surface, that is, walk on the surface although it may still leave a footprint.
2. Description of the Prior Art
There have been various prior art attempts at providing cementitious surface treating compositions and systems meeting the needs of the construction industry, particularly in the protection and waterproofing of concrete surfaces. Such systems should desirably be non-flammable, easy and economical to apply, non-toxic and set within a relatively short period of time into a hard mass or coating with sufficient early strength, abrasion resistance, and corrosion resistance. They should also possess impermeability to fluids, particulary liquids. At the same time, such systems should not possess excessive volume changes under either wet or dry conditions. These types of cementitious systems must also possess positive rather than adverse interaction characteristics with wet unset concrete surfaces. For commercial use the treatment compositions must possess early as well as long term strength and practical field workability. They should be capable of withstanding freezing and thawing as well as the action of salts, solvents and other corrosive substances.
Although there are cementitious concrete surface treatments possessing one or more of the above desirable properties, prior art attempts directed to achieving all of the foregoing in one composition have had only limited success. Many compositions of blended cement demonstrate poor interaction with unset concrete and/or exhibit excessive expansion. Thus, prior art attempts at providing cementitious concrete surface treatment of commercial value in the construction industry have generally been specific or emphasized particular properties in disregard of, or in some instances, in detriment to other desired attributes.
Mixes of Portland cement and high alumina cement are known for their quick-setting nature and this property has been put to practical use on a wide scale. Thus, T. D. Robson, High Alumina Cements and Concretes, John Wiley & Sons, N.Y., 1962, p. 128, discloses that when Portland cement concrete is placed between tides and is subject to early wave action, high alumina cement may be sprinkled on the unset surface and the reaction between the cements quickly gives a hard skin which helps to prevent damage until the main body of the concrete has time to harden.
Prior art cementitious compositions have utilized combination of Portland cement, aluminous cements and gypsums for mixing with water to form masses providing various alleged advantages.
U.S. Pat. No. 3,861,929 discusses the inherent shrinkage characteristics of concretes made with conventional cements which result in cracks on hardening and drying. That patent discloses an expansive cement that, during and after setting and hardening, increases significantly in volume. The expansive cement disclosed comprises a mixture consisting essentially of Portland and calcium aluminate cements and a quantity of calcium sulfate which may be in the form of gypsum. U.S. Pat. No. 3,775,143 discloses a "stressing" cement which includes Portland cement and an expanding component consisting of calcium sulfate, calcium oxide and an aluminate containing material such as aluminous cement or high alkaline calcium hydroaluminate. The cement is said to exhibit waterproofness as well as benzine- and gasproofness.
Compositions containing high alumina cement and Portland cement for use as a quick setting mix have been known. For example, U.S. Pat. No. 4,012,264 states that it is known that very rapidly setting and hardening cement can be produced by mixing Portland cement and high alumina cement and/or by the use of various accelerators and discloses a calcium-aluminate and Portland cement composition including retarders and/or accelerators. High alumina cement has also been added to plaster of Paris or anhydrite plasters in order to give stronger molds or castings. T. D. Robson, High Alumina Cements and Concretes, John Wiley & Sons, N.Y., 1962, pp. 126-7, discloses that lithium salts have been suggested as accelerators for high alumina cement. However, it has been found that compositions containing alumina cement, gypsum and lithium salts as an accelerator, alone, exhibit excessive hardened drying shrinkage. In addition, compositions containing aluminous cement and gypsum alone exhibit excessive hardened wet expansion. Furthermore, addition of Portland cement to such compositions containing aluminous cement and gypsum results in greater hardened wet expansion.
Dry shaking of surface treating constituents on unset concrete has been carried out for years. Such constituents include inert components such as pigments and special aggregates to provide color, abrasion resistance and similar desired properties. In addition, surface treating compositions for dry shaking application have included various types of Portland cement which while not inert are nevertheless compatible with the surface to which they are applied. However, the art has not heretofore used surface treating constituents which would normally be expected to be incompatible with the unset cement surface and to exhibit unwanted volume changes resulting in surface cracking and unstable, self-deteriorating cement.
U.S. Pat. Nos. 4,045,237 and 4,157,263 disclose a cementitious composition possessing a high degree of impermeability to liquid and vapor and reduced long term shrinkage composed of a particulate mixture of Portland cement, a calcined gypsum and high alumina cement. Alternatively, the composition can be composed of a particulate mixture of a high alumina cement and pressure calcined gypsum.
Although not prior art, in my co-pending application Ser. No. 262,175 filed on May 11, 1981 for "Method and Composition For Controlling Volume Change In Fast Setting, Fluid Impermeable Cementitious Systems", I disclose a cementitious composition which, when mixed with water, is capable of setting rapidly into a hard mass of high compressive strength without substantial shrinkage during setting and with reduced wet and dry volume changes in the hardened state while nevertheless possessing a high degree of impermeability to fluids, and abrasion, erosion and chemical resistance and similar characteristics desired in a composition having commercial usefulness in the construction industry. The composition there disclosed comprises a mixture of aluminous cement, gypsum, a drying shrinkage inhibitor preferably Portland cement, and a wet expansion inhibitor, preferably a salt of lithium.